In Exacum and in Saintpaulia the flowers are dimorphic in this sense: the style projects to either the right or the left side of the corolla, from which it follows that a right-handed flower would fertilise a left-handed one, and vice versa.    From Wordnik.com.  [More Letters of Charles Darwin — Volume 2] Reference

Exacum, dimorphism of.    From Wordnik.com.  [More Letters of Charles Darwin — Volume 2] Reference

I noticed Exacum, Careya, Butea arborea, Ficus, Cinchona, Kydia.    From Wordnik.com.  [Journals of Travels in Assam, Burma, Bhootan, Afghanistan and the Neighbouring Countries] Reference